14 
THE GEOLOGIST. 
pear to radiate from aline of imaginary centres, which are coincident 
with the longest axis of the flow ; the inner circumference of these 
radiations being defined by the upper sm-face of the lava-bed, and 
hence the upright planes of columnar crystallization strike at right 
angles downwards fi'om what must have been the primary cooling 
sm-face of the mass, that surface from the fiirst having been slightly 
depressed in its centre. 
5th, The columns which form the " Giant's Causeway" exhibit a 
peculiar beauty and accuracy of form, being in every respect more 
symmetrical than those of any of the other columnar basalts of the 
district, with the one exception of the " Organ-"columns, and on 
this account alone an observer would be led to identify them as be- 
longing to the same bed. 
THE CARBONIFEROUS SYSTEM IN SCOTLAND CHARAC- 
TERIZED BY ITS BRACHIOPODA. 
By Thomas Davidson, Esq., F.R.S., F.G.S., Hon. Member of the 
Geological Society of Glasgow, etc., etc. 
(Gontimiecl from vol. ii., page 4^77.) 
Genus SrmirERA. Sowerby. 1815. 
The shells of which this genus is composed differ much in their external 
shape and appcai'auce, licuce the great diffieulty of correctly dctcrmuiing the 
limits of certain species. Tlie character of this extinct genus are now so weU 
understood that it is scarcely necessary to make any further allusion to the 
subject ; but we may briefly repeat, for the sake of the less initiated, and in 
order to shorten the descriptions of the various species, that aU possess a 
straight hinge-liuc, and a triangular or sub-parallel area, which is chvided by a 
triangular fissure, this last being more or less covered, or contracted, by the 
means of one or two curved plates, to which the term pseudo-deltidium has 
been appUed. The pseudo-deltidium is rarely preserved in the carboniferous 
specimens, but chd ecrtaiidy exist in the perfect or living hichviduals. The vdvcs 
are articulated by the means of curved teeth developed on cither side of the 
fissure in the ventral valve, and which fit into corresponding sockets in the 
opposite or dorsal one. In the larger valve the teeth are supported by vertical 
plates of greater or lesser dimensions, and in the space between these on the 
bottom of the shell are situated the muscular impressions. The adductor, or 
occlusor muscle leaves a narrow mesial oval-shaped sear, and on either side are 
situated the cardinal, or divaricator muscular impressions. In the interior of 
the smaller, or dorsal valve there exists two large conical spiral cods, which 
nearly fill the interior of the shell, the ends bcmg directed outwardly towards 
the cardinal angles, wliilc the bases of the hollow conical spires nearly meet at 
